Nowadays, strut-type suspensions, each composed of a strut and a lower arm are frequently used as suspension apparatuses for vehicles. In general, a strut-type suspension is used for a steered wheel, taking advantage of its structural superiority, in particular. Usually, an A-shaped lower arm, composed of two integral arms is used as a lower link of the strut-type suspension of this kind. One arm of the lower arm is located on the front side in the longitudinal direction of the body of a vehicle, and extends outward from the vehicle body in the transverse direction of the vehicle body. The other arm is located on the rear side in the longitudinal direction of the vehicle body, and extends obliquely to the vehicle body from the outer end of the front arm in the transverse direction of the vehicle body. The wheel is attached to the junction (outer end of the lower arm) between the front and rear arms.
As shown in FIG. 26, front and rear junctions 205 and 206 are respectively provided at the respective inner ends, with respect to the transverse direction of the body of a vehicle, of the front and rear arms of an integral-type A-shaped lower arm 200. The rear junction 206 extends along the longitudinal axis of the vehicle body. The front junction 205 is swingably connected to a pair of brackets 203, 203 that protrude from the vehicle body 202, while the rear junction 206 is supported on the vehicle body 202 by means of a bush 207. Thus, the lower arm 200 is swingable in the vertical direction of the vehicle.
The lower arm 200 can never rotate around both the junctions 205 and 206 at a time. When an external force F.sub.W that acts in the longitudinal direction of the vehicle is applied to the wheel 201, as shown in FIG. 27, therefore, a turning moment acts on the lower arm 200, so that the lower arm 200 rotates around an imaginary pivot or imaginary center of rotation P between the junctions 205 and 206. Forces F.sub.A1 and F.sub.B1 that originate in this tuning moment act on the junctions 205 and 206, respectively, in the directions indicated by the arrows (vectors) in FIG. 27.
At this time, the forces F.sub.A1 and F.sub.B1 act obliquely to the respective axes of the front and rear arms of the lower arm 200. As for the force F.sub.B1, in particular, the force F.sub.B1 acts substantially at right angles to the axis of the rear junction 206, so that it acts to bend the rear arm. The pivot P of the lower arm 200 is situated inside the junctions 205 and 206 in the transverse direction of the vehicle body. In other words, an arm length concerned with the turning moment is greater than the length of the lower arm 200. Accordingly, the turning moment caused by the external force F.sub.W has a large value, and correspondingly, the forces F.sub.A1 and F.sub.B1 are considerably great. Thus, the lower arm 200 is subjected to a very heavy load, which is not desirable.
According to the A-shaped lower arm 200, the transmission of the external force F.sub.W to the vehicle body is eased by utilizing the deflection of the arm. If the arm is too flexible, however, the deflection of the arm becomes excessive when the external force F.sub.W increases as the vehicle turns, for example. This results in a problem that the wheel 201 becomes shaky or the alignment of the toe angle or the like changes unexpectedly, so that the traveling stability of the vehicle is lowered. If the arm is too stiff, on the other hand, the application of the external force F.sub.W causes vibration of the vehicle body.
Proposed in Jpn. Pat. Appln. KOKAI Publication No. 5-270221, etc., therefore, is a split-type A-shaped lower arm 200 that is constructed in the manner shown in FIG. 28, in order to ease the transmission of the external force F.sub.W to the vehicle body satisfactorily and give suitable magnitudes and directions to the forces F.sub.A1 and F.sub.B1. In FIG. 28, the lower arm 200 is composed of a front arm 210 and a rear arm 212 provided separately therefrom. The two arms 210 and 212 are swingably connected to each other. The arms 210 and 212 are connected to the vehicle body 202 at junctions 205 and 206, respectively. The front junction 205 is attached to the brackets 203, 203 of the vehicle body 202 through an elastic bush (not shown). A bush 207, interposed between the rear junction 206 and the vehicle body 202, is formed of a highly elastic material.
According to this split-type lower arm, when the external force F.sub.W acts on the wheel 201, the external force F.sub.W is absorbed satisfactorily by the elastic bushes that are interposed between the vehicle body 202 and the junctions 205 and 206, individually. When the external force F.sub.W acts on the wheel 201, moreover, the front arm 210 is subjected to a force F.sub.A1 that acts substantially in the axial direction of the front arm. Thus, the direction of action of the force F.sub.A1 acting on the front arm 210 is made appropriate.
Even in the split-type A-shaped lower arm 200, however, a force (corresponding to the force F.sub.B1 in FIG. 24), acting on the rear end portion (junction 206) of the rear arm 212 when the external force F.sub.W is applied, acts substantially at right angles to the arm axis at the rear end portion of the rear arm, that is, in the direction in which the arm bends. Accordingly, it cannot be concluded that the load on the lower arm 200 is reduced very much. It is not easy, moreover, to set the rigidity of the junction 206 properly.
In the case where a joint connecting the front and rear arms 210 and 212 is provided on the axis of the front arm 210, as shown in FIG. 28, this joint easily bends when a great external force acts on the wheel 201, especially when the external force acts in the axial direction of the front arm 210, that is, in the transverse direction of the vehicle. If the joint bends, the wheel alignment changes unfavorably.
Described in Jpn. Pat. Appln. KOKAI Publication No. 5-270221 is a tie rod (not shown in FIG. 28), one end of which is connected to the wheel 201 and the other end to the steering. In association with the front arm 210, this tie rod is so designed that the wheel 201 toes out to some extent when the external force F.sub.W acts on the wheel 201 from the front of the vehicle, thereby preventing a tuck-in while the vehicle is turning or being braked. Accordingly, there is also a problem that the traveling stability of the vehicle worsens when the force F.sub.W acts on the vehicle that is traveling ordinarily.
Thus, even the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-270221 cannot solve the problems described above in connection with the integral-type lower arm.